Bleaching pulp



Patented Mar. 26, 1940 UNlTED STATES PATENT OFFICE 2,195,302 BLEACHINGrm Clifford Allen Hampel, Niagara Falls, N. Y., as-

signor to The Mathieson Alkali Works, Inc., New York, N. Y., acorporation of Virginia No Drawing.

pplication January 10, 1939, v

; Serial No. 250,169

' 3 Claims. (Cl. 8-105) lo losic materials with the use of chlorites asbleaching agents which involve the formation of a high density pulphaving a pulp density in excess of about 15% under conditions 'such thatthe chlorite is substantially inactive, and subsequent ll activation ofthe chlorite by increase of temperature, or the addition of acid oracidic materials, or both. Among the specific processes included is thatof forming such a high density pulp containing the chlorite, andactivating the pulp by pass- 20 ing hydrochloric acid gas into contactwith it, advantageously with'heating to temperatures in excess of 50 C.,for example, from 80 C. to 100 C. This invention relates to improvementsin this process. In accordance with the present invention, a highdensity pulp containing the chlorite with a pulp density above about 15%is formed under conditions such that the chlorite is substantiallyinactive, and the chlorite is subsequently acti- 30 vated by bringinginto contact with the pulp a gaseous mixture containing about 25% orless hydrochloric acid gas by volume, for example,

from to 25%, advantageously while heating the pulp above 50 C. Thehydrochloric acid gas is 35 absorbed by the aqueous pulp, and activatesthechlorite, so that the bleach is relatively rapid and complete. By theuse of such a gas mixture, containing less than about 25% ofhydrochloric gas by volume, the concentration of hydrochloric acid 40 inthe aqueous pulp is readily maintained at the optimum concentration foreflective and rapid bleaching, that is, at a concentration correspondingto a pH of about 3.5 to 5.0. This use of the diluted hydrochloric acidgas has important advantages over the use of undiluted hydrochloric acidgas in that it'avoids the formation of local high concentrations of acidwhich might cause local reactions to take place so rapidly thatavailable chlorine is lost before the bleaching is com- 0 pleted, ormight seriously afiect the strength of the cellulosic material bydegradation due to concentrated acid. The use of the dilutedhydrochloric acid gas mixture also has important advantages over the useof aqueous acids for the bleaching of high density pulp. Where aqueousstrong acids are used, the concentration must be so low, to maintain theacidity at the optimum range and to avoid degradation of the fibre, thatthe aqueous acid includes sufiicient water to lower the density of thepulp substantially, with the're- 5 sult that the-bleaching is a lowdensity bleaching, without the important advantages of high densitybleaching with respect to cost, etc.

The chlorites which may be used in bleaching in accordance with thisinvention include the 10" alkali metal chlorites, such as sodiumchlorite, and the alkaline earth metal chlorites, such ,as calcium ormagnesium chlorite. These chlorites are substantially inactive withrespect'to the bleaching of cellulosic materials at temperatures belowabout 50 0., e. g., room temperature, particularly in neutral oralkaline solution, and high density pulps containing the chlorite inproper proportions for bleaching under conditions such thatsubstantially no bleaching occurs are readily prepared. Such highdensity pulps containing the chlorite may be prepared in various ways,as by forming a low density pulp containing chlorite and removing waterfrom it by filtration or expression, or both, by spraying pulp withchlorite g5 solution, etc.

In accordance with the invention, the high density pulp, having adensity in excess of about 15%, e. g., from 15-50%, containing thenecessary amount of chlorite to effect the desired bleach, is

first prepared under conditions of temperature and acidity at whichsubstantially no bleaching occurs, and the pulp is then treated withhydro- 'chloric acid gas diluted with an inert gas, such as air ornitrogen, advantageously while heatedgto temperatures in excess of 0.,for example, from -100 C. or even somewhat higher, to activate thechlorite. The proportion of hydrochloric acid gas in the gas mixtureshould not exceed about 25% by volume, and is advantageouso lyirom 1025%by volume. The amount of the. gas mixture so'used is advantageouslyregulated so the pH of'the pulp is maintained from about 3.5 to 5.0.With many high density pulps it is difiicult or impossible to measurethe pH value; 5 and with such pulps, it is advantageous to supply thegas mixture at a rate such that the desired bleach is obtained in about30-60 minutes, or at a rate such that during the bleaching period, an

amount 01 hydrochloric acid equal to about A 50 the weight of theavailable chlorine or thechlorite is supplied.

- Cellulosic materials generally may be bleached by the process of thisinvention, including paper and paper pulp, such as kratt, sulfite andsulfate 5 pulp, pulped wood; cotton linters, hemp and cellulosicderivatives, such as rayon. Such cellulosic products may be partiallybleached by the use of chlorine, hypochlorites, etc., prior to bleachingathigh pulp densities with chlorites and dilute hydrochloric acid gas.

The invention will be illustrated by the following. specific example,but it is not limited thereto. Example-150 parts (dry basis) or kraftpaper pulp in the form of damp crumbs were stirred into 3000 parts ofwater containing 16.36'parts of available chlorine-as sodium chlorite.The pulp was filtered and pressed to a density of 21.4%. It thencontained 2% of available chlorine based on the dry pulp. The pulp wasshredded, placed in an insulated rotary container heated by live steamat 100 C., and a mixture of hydrochloric acid gas and air, containingabout'12% hydrochloric acid gas, was passed into contact with the pulpat a rate such that about 0.0129 part of hydrochloric acid were suppliedper minute. The

pulp was maintained for about 1 hour at 100 C.

I 1. In thehigh density pulp bleaching of cellulosic material bychlorites, the improvement which comprises activating the chlorite afterformation of a high density pulp having a pulp density in excess ofabout 15% and containing the chlorite under inactive conditions bybringing into contact with the pulp-a mixture of ,an inert gas andhydrochloric acid gas.

2. In the high density pulp bleaching of ce1- .lulosic material bychlorites, the improvement which comprises activating the chlorite afterformation of a high density pulp having a pulp density in excess ofabout 15% and containing the chlorite under inactive conditions bybringing ,.into contact, with the pulp a mixture of an inert gas andhydrochloric acid gas containing up to about 25% of the hydrochloricacid gas by volume -3. In the high density pulp bleaching of kraft pulpby chlorites, the improvement which comiprises activating the chloriteafter formation of a high density kraft pulp having a pulp density inexcess of about 15% and containing the chlorite under inactiveconditions by bringing into contact withithe pulp a mixture of an inertgas and hydrochloric acid gas containing up to about 25% of thehydrochloric acid gas by volume.

CLIFFORD ALLEN HAMPEL. a0

